You can take a look into the source-code of one of your article-urls. There will be automatically added the link-rel-amphtml meta-tag:
<link rel="amphtml" src="https://yourdomain.com/yourarticle/?amp=1">
This tag would be seen from search-bots like Google. So if Google decided to cache and index this AMP-URL it would be shown in search-results, if a user uses a mobile device and Google thinks its better to display the amp-version instead the non-amp-version.
So you havent to add it manually, if you come from Google-Search (and Google had indexed your amp-version).
If you come from other channels, like direct visit, you have to add it manually.
If you want to redirect every user only to your amp-version, your have to redirect them to the ?amp=1-url.
PageSpeed Insights analyze just the url you have inserted. To analyze the amp-version your have to add the “?amp=1”.